Prediction of permeation properties of CO2 and N2 through silicalite via molecular simulations

The sorption isotherms and self-diffusivities of CO2 and N-2 in silicalite have been calculated via grand canonical Monte Carlo and equilibrium molecular dynamics simulations over a wide range of occupancies, using various force fields proposed in the literature. Predictions for the sorption thermodynamics are in very favorable agreement with the experiment, especially when detailed point-charge models are used to represent the interaction of the quadrupole moments of the sorbate molecules with the lattice field and with each other. They indicate that the zeolite cannot be in its para (P2(1)2(1)2(1)) form under the conditions of the measurements. Permeabilities corresponding to a perfectly crystalline membrane have been estimated for CO2 and N-2, as well as for methane, examined in past simulation work, from the predicted sorption isotherms and low-occupancy self-diffusivities by invoking the Darken equation. The ratios of pure component permeabilities obtained in this way agree very well with actual macroscopic values obtained from carrying out permeation measurements for the different pure sorbates in the same silicalite membrane. Absolute magnitudes of the permeabilities, however, exceed by more than 2 orders of magnitude the reported macroscopic values, which themselves vary widely among different experimental investigations. The large, morphology-dependent nonuniformity in membrane thickness of actual supported silicalite membranes is proposed as a plausible reason for this disparity.